OF THE UNITED STATES. 261 



To prepare for reading;, the microscopes were placed over 

 the decimals, which, on the scale, corresponded to the length 

 of two metres, viz. 78",7 or 78",8 ; and this distance was 

 taken from +(" to 79",8 ; whicli brought it equidistant from 

 both ends of the scale. The microscopes were fixed to 

 this distance upon the scale with the greatest care ; were 

 then left for some time and were again verified. The scale 

 was removed, and the two metres, with their supports, pro- 

 perly laid in its place, so as to bring the middle of their 

 breadth under the faces of tiie microscopes. The middle 

 contact was exactly made, the butting pieces laid to botli 

 ends, the coincidence of the end under the microscope with 

 fixed wires effected, the middle contact again verified with 

 a magnifying glass, tlie moveable wires of the micrometer 

 microscope moved bytiie micrometer screw upon the image 

 of the contact under its focus, and the value of the corre- 

 sponding subdivisions read on the micrometer by its revo- 

 lutions and subdivisions. 



The longitudinal motion required to effect the contact can- 

 not be communicated by the hand alone : the best mode of 

 communicating tins motion is by a few light strokes of a 

 suitai)le piece of wood, applied carefully, and in such a man- 

 ner as not to separate the different pieces by the counter- 

 stroke. 



Tiie value oftlie micrometer parts was ascertained by re- 

 peated measurements of a decimal in difi'erent parts of the 

 scale. From a mean of many such measurements, with the 

 adjustment of tlie microscopes used for the metres, I found 

 one decimal on the scale to be measured by one decimal 

 and four units of the micrometer, or 0",l of the scale equal 

 to 0",1004 of the micrometer. 



Lastly was to be determined, the individual value of the 

 distance on the scale used in the comparison, in relation to 

 the moan value of the same distance, resulting from its mea- 

 surement taken on as many parts of the scale as were ad- 

 missible, in order to give to the standards compared their 



VOL. II. — M 2 



